Conventional acoustic logging of earth formations traversed by a liquid-filled borehole is accomplished by lowering into the borehole a logging tool suspended on an armored communication cable. The typical logging tool will usually incorporate several acoustic transducers. At least one transducer will be used as a transmitter to generate acoustic signals which are to be detected by one or more transducers that act as receivers. The desired detected signals would be representative of the energy from the transmitter which travels through the borehole or the surrounding formation to the detector and it would not contain anything else, such as, a "tool-mode noise" or a "road noise" which will be discussed hereinafter.
The acoustic signal generated by the transmitter centered in the borehole can be a symmetrical or an asymmetrical compressional waveform with respect to the borehole axis in the fluid. When the generated compressional wave travels through the liquid in the borehole and strikes the borehole wall, various types of elastic and guided waves, which will be referred to herein as borehole waves, are produced as the earth-borehole response to the generated signals. The types of borehole waves produced have different velocities and amplitude-frequency characteristics. Since these borehole waves are usually detected at a receiver transducer through a fluid coupling, borehole waves will also be referred to herein as acoustic waves or signals. Because these acoustic wave types have different velocities and characteristics, various methods are used to enhance the ability of the logging tool to detect the waveforms so that the wave types may be distinguished during processing. Through the processing of these waveforms, particularly through the amplitude and phase relationships of the waveforms as a function of time and distance, the viscoelastic properties of the earth formations surrounding the borehole can be deduced, such as, in particular, the compressional and shear wave speeds of the earth formations.
Transducers used as receiver elements (also referred to herein as transducer detectors) may be combined to form a receiver station. One type of transducer, a piezoelectric transducer, has been used for the receiver element. Prior art (as one example, U.S. Pat. No. 4,649,526) has taught: the use of multipole logging tools in subsurface exploration, the addition/subtraction of the output of a plurality of detector transducers to form one combined signal (a composite signal) for each receiver station, and the use of piezoelectric elements for detector transducers. The combination of outputs can take several forms or modes. The selective addition and/or subtraction of signals to form the composite signals from specifically located piezoelectric detector elements at each receiver station is used to detect various borehole-propagation modes. When used in this manner, the following modes of borehole propagation can be detected: monopole, dipole, quadrupole, octopole and other borehole-propagation modes which may be initiated by selected multipole transmitters.
A logging tool generally consists of several receiver stations, spaced at some interval along the body of the tool. The collection of receiver stations will be referred to herein as a receiver array.
Preferably, the data collected by the receiver will only include an accurate representation of the earth (or the borehole) response to the signals generated from the transmitters.
However, signals obtained from conventional acoustic multipole logging tools are subject to various noises such as "tool-mode noise" and "road noise" and they may also be affected by transducer detector resonance effects. Either of these problems limits the ability of the tools to obtain signals from the detector transducers which accurately and only represent the response of the borehole environment to the various propagation modes of the generated signals.